Problem: In her chemistry class, Tiffany took 4 tests. Her scores were 81, 94, 90, and 91. What was her average score on the tests?
Explanation: The average is the sum of her scores divided by the number of scores. There are 4 scores and their sum is $81 + 94 + 90 + 91 = 356$ Her average score is $356 \div 4 = 89$.